The present invention generally relates to electric lamps and methods of manufacture. More specifically, the present invention relates to lamps wherein the light source includes a light emitting plasma contained within an arc tube (i.e. plasma lamps) having dichroic thin film coatings to improve the operating characteristics of the lamp.
Plasma lamps such as mercury lamps or metal halide lamps have found widespread acceptance in lighting large outdoor and indoor areas such as athletic stadiums, gymnasiums, warehouses, parking facilities, and the like, because of the relatively high efficiency, compact size, and low maintenance of plasma lamps when compared to other lamp types. A typical plasma lamp includes an arc tube forming a chamber with a pair of spaced apart electrodes. The chamber typically contains a fill gas, mercury, and other material such as one or more metal halides, which are vaporized during operation of the lamp to form a light emitting plasma. The operating characteristics of the lamp such as spectral emission, lumens per watt (“LPW”), correlated color temperature (“CCT”), and color rendering index (“CRI”) are determined at least in part by the content of the lamp fill material.
The use of plasma lamps for some applications has been limited due the difficulty in realizing the desired spectral emission characteristics of the light emitting plasma. For example, metal halide lamps were introduced in the United States in the early 1960's and have been used successfully in many commercial and industrial applications because of the high efficiency and long life of such lamps compared to other light sources. However, metal halide lamps have not as yet found widespread use in general interior retail and display lighting applications because of the difficulty in obtaining a spectral emission from such lamps within the desired range of CCT of about 300-400 K and CRI of greater than about 80.
Relatively high CRI (>80) has been realized in metal halide lamps having a CCT in the desired range by the selection of various metal halide combinations comprising the lamp fill material. For example, U.S. Pat. No. 5,694,002 to Krasko et al. discloses a metal halide lamp having a quartz arc tube with a fill of halides of sodium, scandium, lithium, and rare earth metals, which operates at a CCT of about 300 K and a CRI of about 85. U.S. Pat. No. 5,751,111 to Stoffels et al. discloses a metal halide lamp having a ceramic arc tube with a fill of halides of sodium, thallium and rare earth metals which operates at a CCT of about 300 K and a CRI of about 82. However, the quartz lamps disclosed by Krasko et al. have a relatively low LPW, the ceramic lamps disclosed by Stoffels et al. are relatively expensive to produce, and both types of lamps have a relatively high variability in operating parameters and a relatively diminished useful operating life.
The use of a sodium/scandium based halide fill in plasma lamps has addressed the efficiency and variability problems by providing improved efficiency and lower variability in operating parameters relative to metal halide lamps having other fill materials. However, such lamps have a relatively low CRI of about 65-70 and thus are not suitable for many applications.
One known approach in improving certain operating characteristics of plasma lamps is to filter the light emitted from the plasma. Recent developments in thin film coating technology have increased the utility of such coatings in the lighting industry by improving both the thermal capability of the coatings and the uniformity of such coatings when applied to curved surfaces such as the arc tubes, reflectors, and outer envelopes of lamps. The MicroDyn® reactive sputtering process of Deposition Sciences, Inc. of Santa Rosa, Calif., as disclosed and claimed for example in U.S. Pat. No. 5,849,162 is particularly suitable for depositing a variety of thin film coatings useful in lighting applications. Other known coating processes such as chemical vapor deposition, thermal evaporation, and ion and electron beam deposition may also be suitable for lighting applications.
It is a characteristic of such coatings that they selectively reflect and/or absorb radiation at selected wavelengths. For example, U.S. Pat. No. 5,552,671 to Parham et al. discloses a multilayer UV radiation absorbing coating on the arc tubes of metal halide lamps to block UV radiation. U.S. Pat. No. 5,646,472 to Horikoshi discloses a metal halide lamp having a dysprosium based fill with a multilayer coating on the arc tube for reflecting light at wavelengths shorter than nearly 600 nm while transmitting light at longer wavelengths to lower the CCT of the lamp. However, the optimal utilization of thin film coatings to control certain operating characteristics of plasma lamps often requires that a significant portion of the light that is selectively reflected by the coating be absorbed by the plasma, and there remains a need for thin film coatings for plasma lamps directed to plasma absorption.
It is accordingly an object of the present invention to obviate many of the deficiencies of the prior art and to specifically address the plasma absorption of reflected light in the improvement of the operating characteristics of plasma lamps.
Another object of the present invention is to improve the effectiveness of thin film coatings used in plasma lamps by consideration of the absorption of reflected light in the plasma in the design and fabrication of such coatings.
Still another object of the present invention is to provide a novel multilayer thin film filter and method for plasma lamps.
Yet another object of the present invention is to provide a novel plasma lamp with improved operating characteristics and method of manufacturing such plasma lamps.
Still yet another object of the present invention to provide a novel plasma lamp and method using multilayer thin film coatings to obtain the desired spectral emission characteristics for the lamp.
A further object of the present invention is to provide a novel plasma lamp and method of making plasma lamp with operating characteristics suitable for indoor retail and display lighting.
Yet a further object of the present invention to provide a novel metal halide lamp and method having a highly selective notch in transmissivity.
Still a further object of the present invention to provide a novel method of making multilayer thin film coatings for plasma lamps wherein the number and thickness of the layers in the coating are determined as a function of the spectral and/or physical characteristics of the plasma.
Yet still a further object of the present invention to provide a novel method of making multilayer thin film coatings for plasma lamps wherein the number and thickness of the layers in the coating are determined as a function of the geometry of the surface to be coated and/or and angular distribution of the light emitted from the plasma on the coating.
It is still another object of the present invention to provide a novel sodium/scandium lamp and method.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.